A delegation from a convention of the National Religious Broadcasters, an evangelical church organization that includes followers of Jerry Falwell and Billy Graham, dropped by the Israeli Embassy the other day for a briefing. It included a special performance by actress Rosina Fernhoff from Tel Aviv.

Her moving monologue, in which she plays the part of a teacher in an Israeli school seized by Palestinian terrorists, powerfully expresses the torment on both sides of the Arab- Israeli conflict. But the briefing officer had the air of a man with higher priorities. He left no doubt he would have liked to have had more time to enlarge on Israel's immediate security concerns--and the one in particular that Israel can do something about.

The message he (and the Israeli government) wanted to broadcast to Americans is that the Palestine Liberation Organization is in blatant violation of the cease-fire across the Israeli-Lebanese frontier--a cease- fire that was engineered last summer through the good offices of American special envoy Philip Habib. A flurry of news reports originating from Israel has begun to hammer the same point.

What is at work here, quite obviously, is an orchestrated campaign to engage American understanding and support if--or when--the gung- ho Israeli military establishment prevails and Prime Minister Menachem Begin agrees to a full-scale Israeli invasion to crush the rapidly expanding PLO military forces in southern Lebanon.

It is never easy in these matters to distinguish warnings issued for deterrence's sake from signals of flat-out intent. My guess is that in this case the distinction isn't worth making. The current Lebanese scene is quite sufficiently incendiary to serve as a hair-raising reminder of how little it could take, in the absence of a serious and credible peace process, to spark from one side or the other a resort to violent measures.

The Israelis have amassed a heavy concentration of forces just south of the Lebanese border. At the embassy here, officials are quick to produce persuasive intelligence reports on the degree to which the PLO has exploited the cease-fire to expand its striking power.

By Israeli reckoning, since last summer the PLO in Lebanon has received 1,000 tons of military equipment directly from the Russians, 200 tons from Saudi Arabia and 100 tons from Libya, including ground-to-air-missiles. The number of ground-to-ground missiles purportedly has more than doubled; tank strength has grown from 34 to 70, and been upgraded to more powerful models.

The Israelis claim that the PLO has been end-running the Lebanese cease-fire with 14 separate infiltration attempts into Israel from Jordan, at a cost of seven Israelis killed and 49 wounded. Some 29 terrorist incidents aimed at Israelis in third countries have taken nine lives.

"We regard all this as violations of the cease-fire," says an Israeli spokesman. He quickly adds that it does "not yet" constitute the "clear provocation" Begin has spoken of in defining what it would take to trigger an Israeli plunge into Lebanon. But the emphasis is on "not yet." As last year's Israeli move on Golan and Israel's bombings of Baghdad and Beirut would suggest, it would be unwise to count on just when a PLO "provocation" might become "clear" to Menachem Begin.

The PLO, for its part, is torn by dissent between fanatic and more moderate factions, not necessarily subject to anybody's restraining hand for any predictable period of time.

With all this presumably in mind, the Reagan administration is sending Habib back to the scene. His presence is a proven pacifier. It may be enough to damp things down at least until the deadline that is at the center of concern: April 25, when Israel is scheduled to complete its final withdrawal from the Sinai and the Israeli-Egyptian peace treaty comes into full force.

But if that's the extent of the Habib mission, how does the Reagan administration propose to keep the Middle East peace after April 25? What's left of Camp David doesn't meet the need. The choice of an amiable, but unknown and unknowledgeable U.S. special negotiator for the Palestinian "autonomy" talks scarcely lends weight or urgency to the proceedings. Secretary of Defense Caspar W. Weinberger's dabbling in security arrangements with Saudi Arabia and arms deliveries to Jordan can only serve to add to Israel's sense of abandonment and impulse to go it alone.

What looms ahead is an American policy void--a nothingness of the sort that historically has given free play to violent, unilateral acts. Lebanon is the likeliest ignition-point.